You could run DRC-121 with 'fake' requirements to have inter-page (flat) and intra-page (internal) symbols. This would flag all 'connected' nets as not meeting the requirements. See this post on DRC-121 and the other posts referenced by it http://communities.mentor.com/thread/14537?tstart=0

We are using the same symbols for internal connections (same sheet) and flat connections (other sheets) and so I get many Extra internal connection symbol on netandRedundant flat symbol on netmessages. But looking for "missing symbols" in the drc-121 messages I can find the relevant warnings.

Well, at least for the top level schematics. As I wrote in another discussion (http://communities.mentor.com/thread/14555), for me DRC checks only one level of the schematic, it does not go into the lower levels. When I manually check the lower levels, I get many "missing" messages for nets which go to the upper level and so have port symbols instead of internal or flat symbols.

"You should also define the 'Port' symbols for completeness."I have defined the Hierarchical input symbol(s), Hierarchical output symbol(s) and Hierarchical bidirectional symbol(s) in the rules for the drc-121.

But my description was incomplete. I get the "missing symbols" warnings for nets which are both connected within the lower block and to the upper level. In my schematic these nets have only ports, but no internal or flat connectors, and obviously DRC misses the internal or flat connectors.

Is my usage of the ports and connectors wrong or is DRC not smart enough here?

You need to provide an example where you get the " "missing symbols" warnings for nets which are both connected within the lower block and to the upper level" otherwise we're second guessing where the issue lies.